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Advances in our understanding of global hydrological processes will require detailed precipitation estimates on a broad
range of time and space scales. Satellite observations provide a critical contribution toward mapping global rainfall and its
variability. Over long time periods, monthly records of precipitation will prove valuable in determining global and regional
precipitation trends and possibly separating anthropogenic changes from the large natural variations in rainfall. On shorter time
and space scales, global maps of rainfall and latent heating structure will prove very useful as validation tools for general
circulation models as they attempt to forecast climate conditions. Assimilation of the precipitation information into global and
regional models should produce more realistic simulations. On even smaller time and space scales, knowledge of surface rainfall
will be useful for the improvement of surface hydrology models.
ER-2 Doppler Radar
The ER-2 Doppler Radar (EDOP) aboard the high-altitude ER-2 aircraft is a dual-beam 9.6 GHz radar to measure
reflectivity and wind structure in precipitation systems. Beginning in 1993 EDOP has obtained reflectivity and Doppler wind
measurements from a variety of mesoscale precipitation systems, including a classic squall line over the Gulf of Mexico, which
had an extensive trailing stratiform region, and the remnants of Tropical Storm Jerry. These data sets have provided valuable
information on the structure of precipitation systems and data sets for the testing of spaceborne rain estimation algorithms.
 Figure 1 shows an example of EDOP reflectivity and Doppler velocity observations taken from a stratiform rain region.
The reflectivity (upper) panel shows a weak radar "bright band" at approximately 4.5 km altitude and peak reflectivities in the
rain layer of about 45 dBZ. The Doppler velocity (lower) panel which provides the vertical motions of the hydrometeors
(positive values are downward motions), shows low fallspeeds in the upper snow region (yellow and green shades) and higher
fallspeeds characteristic of rain below the bright band (red shades). Also, note a strong convective cell embedded in the
stratiform rain (blue region between 20 and 40 km distance).
During some flights, EDOP flew in conjunction with a number of other radiometric instruments covering frequencies from
the visible to high-frequency microwaves. The combined radar/radiometer data sets from these flights have been used to
develop rain estimation algorithms for the Tropical Rainfall Measuring Mission (TRMM). Studies in the near future will be
performed on the radar/radiometer observations to provide further understanding of the structure of rain systems and to
improve the TRMM rain algorithms. After TRMM launch the EDOP radar will be used to validate measurements from the
Precipitation Radar (PR) on TRMM and provide detailed rainfall and dynamical information for comparison with TRMM
observations.
Physical Retrievals of Rainrate and Structure
Estimating the surface rainfall and the associated vertical hydrometeor structure from satellite information is important
because these quantities determine the absolute magnitude and the shape of the latent heating profiles in the atmosphere. To
retrieve these parameters, a physical retrieval approach has been developed that makes use of cloud dynamical models such as
the Goddard Cumulus Ensemble model to establish prior probability densities of precipitation structures. Detailed radiative
transfer computations through the model fields are compared with aircraft and satellite observations to determine the most likely
profiles . A sample result of the Goddard Profiling Algorithm (GPROF)
applied to passive microwave radiometer data flown on the NASA ER-2 aircraft is shown in Figure 2. The top panel
shows the aircraft-observed brightness temperatures. The center panel shows the retrieved hydrometeor profiles derived from application of
the GPROF algorithm to the aircraft data. The bottom panel shows observed reflectivity structure measured by the EDOP radar flying on the
same aircraft. The retrieved rainfall information is converted to equivalent reflectivities for easier comparison with the measured
reflectivities. The results show that this approach is successful at determining the surface rainfall rate and the varying characteristics of the vertical
hydrometeor structure. This algorithm is being applied to current passive microwave satellite observations to produce
instantaneous and monthly maps of surface precipitation and vertical structure characteristics. In the near future this approach
will be used in analyzing data from the TRMM and from the Advanced Microwave Scanning Radiometer (AMSR) to fly on the
EOS-PM satellite and on the Japanese ADEOS-2 satellite.
Rainfall from Tropical Cyclones
Since tropical cyclones can release large amounts of latent heat, quantitative information concerning the role of these
systems in distributing rainfall is critical to understanding the impact that tropical cyclones have in altering the general circulation
and climate of our planet. Rainfall from tropical cyclones derived from Special Sensor Microwave/Imager (SSM/I) data were
compared to rainfall totals from all raining systems.
The bottom panel of Figure 3, which delineates the total accumulated rainfall during a 35 month period, indicates that
the regions with the greatest rainfall (greater than 300 mm mo-1) are associated with the ascending branch of the Hadley
circulation in the ITCZ and within the western North Pacific monsoon trough region. The middle panel of Figure 3, which
depicts the accumulated rainfall contributed by tropical cyclones, indicates that the regions where tropical cyclone rainfall is the
greatest (i.e., greater than 35 mm mo-1) are mainly located in the subtropical regions (i.e., 10 - 25 N) of the western and eastern North Pacific. However, it is noted from the percentage of rainfall contributed by tropical cyclones (top panel of Figure 3
), that the regions where tropical cyclones contribute the greatest percentage of rainfall are slightly north of the regions where
maximum tropical cyclone rainfall accumulation occurs. Within the western North Pacific, the maximum percentage of rainfall
contributed by tropical cyclones is approximately 30% and is located northeast of the Philippines Islands.
Regional Rainfall Climatologies
Knowledge of the time and space patterns of climate-scale precipitation is important in the diagnosis of the global climate
system, including the general circulation. Observations from space are crucial in producing global estimates because large areas
of the globe are characterized by non-existent or inadequate surface data. The region of Amazonia is one such example.
Figure 4a shows the results of the GSCAT microwave technique (based on the scattering of radiation at 86 GHz)
applied to data from the SSM/I on all overpasses in 1988, in mm/year. Maxima in rainfall occur in the ITCZ over the Atlantic
Ocean along 5 N, near the mouth of the Amazon River. near Belem, and at various places between the major rivers of the
Amazon. The latter are the result of terrain-induced local circulations. The region of northeast Brazil is shown to be particularly
dry.
To answer the question, "Is there a preference for morning or evening rainfall over Amazonia?", the difference between
the rainfall estimates from the afternoon and early morning orbits of the SSM/I is computed. Results are shown in Figure 4b
with morning maxima displayed using blue shades and afternoon maxima displayed using red/orange/yellow shading.
Morning maxima appear predominantly over the ocean, and just offshore of the northern coast of Brazil. Concavities in the coastline tend
to have higher values due to increased local convergence. Morning maxima also appear along the rivers of Amazonia,
particularly at the confluence of the Negro and Solimoes Rivers. Afternoon maxima appear along the entire length of the
northern coast, the result of an extended land-sea breeze circulation. These squall lines migrate inland, creating an alternating
pattern of morning and evening maxima in precipitation.
Global Precipitation Estimates
To achieve the best possible analysis of the global distribution of precipitation, information from passive microwave
sensors in low-orbit, infrared sensors in geosynchronous orbit and surface-based raingauge networks are combined to
produced a merged global precipitation analysis. The currently-available data set covers the period July 1987 through
December 1994 on a monthly 2.5 x2.5 latitude/longitude grid. Error estimates are provided with each precipitation field, based
on sampling considerations. These (spatially and temporally varying) errors allow users to assess the utility of the grid values for
their own application. These data sets are suitable for a wide range of applications, including validation of numerical
climatological models, calibration of hydrological models, and benchmarking of experimental rainfall estimation techniques.
These analyses are part of the Global Precipitation Climatology Project (GPCP).
Figure 5 (top) shows the precipitation (mm/mo) for August 1987. The middle panelshows the precipitation averaged
over the 7-year period 1988-1994 while the bottom panel displays the seasonal difference between the 7-year averages of
June/July/Aug. and Dec./Jan./Feb.
Ocean Surface Fluxes
The surface fluxes of momentum and sensible and latent heat over the global oceans are essential to weather, climate and
ocean problems. Evaporation is an important component of the atmospheric hydrological cycle and the surface heat budget; the
wind stress is the primary driving force for the oceanic circulations. These fluxes over the global oceans are required to drive
ocean models and to validate coupled ocean-atmosphere global models. The freshwater flux (precipitation minus evaporation)
is important for the oceanic mixed-layer model. In addition, the daily and monthly fluxes can be used to improve understanding
of air-sea interactions and physical processes over the western Pacific warm pool and other critical air-sea interaction locations.
A satellite algorithm has been developed to retrieve daily surface fluxes of momentum and sensible and latent
heat over global oceans from the SSM/I data using a stability-dependent bulk scheme. Daily latent heat fluxes (and wind
stresses and sensible heat fluxes) are computed from the daily SSM/I surface wind speeds and the daily SSM/I surface
humidity, the daily NCEP (National Centers for Environmental Prediction) sea surface temperatures, and the daily ECMWF
(European Centre for Medium-Range Weather Forecast) analyzed 2-m temperatures. Figure 6 shows monthly averages of the
retrieved daily SSM/I (a) latent heat fluxes and (b) wind stresses for February 1993. Arrows indicate SSM/I vector winds. In
addition, the retrievals have been used to study the impacts of westerly wind bursts and associated super cloud clusters on
evaporative cooling and the sea surface temperature over the equatorial Indian and Pacific oceans.
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Figure 1 shows an example of EDOP reflectivity and Doppler velocity observations taken from a stratiform rain region.
The reflectivity (upper) panel shows a weak radar "bright band" at approximately 4.5 km altitude and peak reflectivities in the
rain layer of about 45 dBZ. The Doppler velocity (lower) panel which provides the vertical motions of the hydrometeors
(positive values are downward motions), shows low fallspeeds in the upper snow region (yellow and green shades) and higher
fallspeeds characteristic of rain below the bright band (red shades). Also, note a strong convective cell embedded in the
stratiform rain (blue region between 20 and 40 km distance).
During some flights, EDOP flew in conjunction with a number of other radiometric instruments covering frequencies from
the visible to high-frequency microwaves. The combined radar/radiometer data sets from these flights have been used to
develop rain estimation algorithms for the Tropical Rainfall Measuring Mission (TRMM). Studies in the near future will be
performed on the radar/radiometer observations to provide further understanding of the structure of rain systems and to
improve the TRMM rain algorithms. After TRMM launch the EDOP radar will be used to validate measurements from the
Precipitation Radar (PR) on TRMM and provide detailed rainfall and dynamical information for comparison with TRMM
observations.
. A sample result of the Goddard Profiling Algorithm (GPROF)
applied to passive microwave radiometer data flown on the NASA ER-2 aircraft is shown in Figure 2. The top panel
shows the aircraft-observed brightness temperatures. The center panel shows the retrieved hydrometeor profiles derived from application of
the GPROF algorithm to the aircraft data. The bottom panel shows observed reflectivity structure measured by the EDOP radar flying on the
same aircraft. The retrieved rainfall information is converted to equivalent reflectivities for easier comparison with the measured
reflectivities. The results show that this approach is successful at determining the surface rainfall rate and the varying characteristics of the vertical
hydrometeor structure. This algorithm is being applied to current passive microwave satellite observations to produce
instantaneous and monthly maps of surface precipitation and vertical structure characteristics. In the near future this approach
will be used in analyzing data from the TRMM and from the Advanced Microwave Scanning Radiometer (AMSR) to fly on the
EOS-PM satellite and on the Japanese ADEOS-2 satellite.
The bottom panel of Figure 3, which delineates the total accumulated rainfall during a 35 month period, indicates that
the regions with the greatest rainfall (greater than 300 mm mo-1) are associated with the ascending branch of the Hadley
circulation in the ITCZ and within the western North Pacific monsoon trough region. The middle panel of Figure 3, which
depicts the accumulated rainfall contributed by tropical cyclones, indicates that the regions where tropical cyclone rainfall is the
greatest (i.e., greater than 35 mm mo-1) are mainly located in the subtropical regions (i.e., 10 - 25 N) of the western and eastern North Pacific. However, it is noted from the percentage of rainfall contributed by tropical cyclones (top panel of Figure 3
), that the regions where tropical cyclones contribute the greatest percentage of rainfall are slightly north of the regions where
maximum tropical cyclone rainfall accumulation occurs. Within the western North Pacific, the maximum percentage of rainfall
contributed by tropical cyclones is approximately 30% and is located northeast of the Philippines Islands.
Knowledge of the time and space patterns of climate-scale precipitation is important in the diagnosis of the global climate
system, including the general circulation. Observations from space are crucial in producing global estimates because large areas
of the globe are characterized by non-existent or inadequate surface data. The region of Amazonia is one such example.
Figure 4a shows the results of the GSCAT microwave technique (based on the scattering of radiation at 86 GHz)
applied to data from the SSM/I on all overpasses in 1988, in mm/year. Maxima in rainfall occur in the ITCZ over the Atlantic
Ocean along 5 N, near the mouth of the Amazon River. near Belem, and at various places between the major rivers of the
Amazon. The latter are the result of terrain-induced local circulations. The region of northeast Brazil is shown to be particularly
dry.
To answer the question, "Is there a preference for morning or evening rainfall over Amazonia?", the difference between
the rainfall estimates from the afternoon and early morning orbits of the SSM/I is computed. Results are shown in Figure 4b
with morning maxima displayed using blue shades and afternoon maxima displayed using red/orange/yellow shading.
Morning maxima appear predominantly over the ocean, and just offshore of the northern coast of Brazil. Concavities in the coastline tend
to have higher values due to increased local convergence. Morning maxima also appear along the rivers of Amazonia,
particularly at the confluence of the Negro and Solimoes Rivers. Afternoon maxima appear along the entire length of the
northern coast, the result of an extended land-sea breeze circulation. These squall lines migrate inland, creating an alternating
pattern of morning and evening maxima in precipitation.
To achieve the best possible analysis of the global distribution of precipitation, information from passive microwave
sensors in low-orbit, infrared sensors in geosynchronous orbit and surface-based raingauge networks are combined to
produced a merged global precipitation analysis. The currently-available data set covers the period July 1987 through
December 1994 on a monthly 2.5 x2.5 latitude/longitude grid. Error estimates are provided with each precipitation field, based
on sampling considerations. These (spatially and temporally varying) errors allow users to assess the utility of the grid values for
their own application. These data sets are suitable for a wide range of applications, including validation of numerical
climatological models, calibration of hydrological models, and benchmarking of experimental rainfall estimation techniques.
These analyses are part of the Global Precipitation Climatology Project (GPCP).
Figure 5 (top) shows the precipitation (mm/mo) for August 1987. The middle panelshows the precipitation averaged
over the 7-year period 1988-1994 while the bottom panel displays the seasonal difference between the 7-year averages of
June/July/Aug. and Dec./Jan./Feb.
A satellite algorithm has been developed to retrieve daily surface fluxes of momentum and sensible and latent
heat over global oceans from the SSM/I data using a stability-dependent bulk scheme. Daily latent heat fluxes (and wind
stresses and sensible heat fluxes) are computed from the daily SSM/I surface wind speeds and the daily SSM/I surface
humidity, the daily NCEP (National Centers for Environmental Prediction) sea surface temperatures, and the daily ECMWF
(European Centre for Medium-Range Weather Forecast) analyzed 2-m temperatures. Figure 6 shows monthly averages of the
retrieved daily SSM/I (a) latent heat fluxes and (b) wind stresses for February 1993. Arrows indicate SSM/I vector winds. In
addition, the retrievals have been used to study the impacts of westerly wind bursts and associated super cloud clusters on
evaporative cooling and the sea surface temperature over the equatorial Indian and Pacific oceans.